Excessive ultraviolet (UV) radiation is harmful to life. The damage caused to the biological systems by ultraviolet radiation is by

Although UV photons of different energies have various effects on DNA, the most impoant damage to DNA is the formation of pyrimidine dimers. In the pyrimidine dimer, two adjacent pyrimidine bases--cytosine (C) and/or thymine (T)--are linked in an abnormal structure which distos the shape of the DNA double helix and blocks its copying by the DNA replication or RNA transcription machinery. A block in either of these impoant processes would be very dangerous for a cell; as little as one dimer per cell in fact, can be lethal. Dimers are formed in DNA most efficiently by UV-C, less efficiently by UV-B, and very little by UV-A action.